The efforts towards higher efficiency promoting the use of lesser numbers of blade have led several studies to illustrate the flow physics in a compressor blade passage. The lack of certainty in designing an axial compressor blade lies in the nature of its flow. In a compressor cascade, the boundary layer past the rotor or stator blade is subjected to an adverse pressure gradient. Further at high altitudes and low flight speeds, Reynolds number (*Re*) becomes low deteriorating the performance of a compressor stage. Under these conditions, flow remains laminar over a large percentage of the blade chord until it separates from the surface. The separated boundary layer being unconditionally unstable, initiates a rapid transition and then it reattaches as a turbulent boundary layer.

      There are numbers of study to improve the compressor performance and efficiency by changing shapes of the blade. Airfoil shapes, that control the diffusion of velocity over the surface, can enhance the amount of laminar flow in relation to turbulent flow. This also may delay or avoid flow separation even before the trailing edge is reached. Thus, the aerodynamic loading becomes higher for a Controlled-Diffusion (C-D) blade as compared to the conventional one. In the present study, a high fidelity Large-Eddy Simulation (LES) with dynamic subgrid-scale model is used to predict the flow field on a 67B C-D compressor blade at *Re* of 2.1E05 based on the chord and the inlet velocity. An in-house LES solver, where the governing equations are in covariant form, is used on a staggered mesh of 8.5 million. The solver has been extensively validated for variety of transitional and turbulent flows. When compared with the experiment, LES results depict a separation bubble in the mid-chord region of the suction surface, although discrepencies exist. Further, the LES resolves the growth of boundary layer near the mid-chord and levels of turbulence intensity with an acceptable limit. What is noteworthy that LES also resolves a tiny separation bubble near the leading edge on the suction surface at the designed inflow angle of 38.3 deg. The objective of the present study is to assess how this leading edge bubble influences the transition and development of downstream boundary layer. To resolve this, the flow features and the underlying transition mechanisms on the blade at incidences of ± 5 deg are also presented.

            The boundary layer becomes pre-transitional because of appearance of a separation bubble near the leading edge followed by a decay of fluctuations up to 30% chord attributing to the local flow acceleration on the suction surface. Further downstream, boundary layer appears laminar after being relaxed from the leading edge excitation, where the shape factor becomes 2.3. In the mid-chord region of the blade, the boundary layer separates again because of the adverse pressure gradient and initiates transition followed by breakdown. With an incidence of +5 deg, the effect of leading edge contamination is amplified with formation of a larger bubble exciting boundary layer on the suction surface. The decay of fluctuations is also evident in this case, but the flow becomes transitional near the mid-chord, resulting in a reduction of the bubble length. An incidence -5 deg, promotes a leading edge separation on the pressure surface, whereas the leading edge separation on the suction side disappears. In brief, LES resolves the contaminations of boundary layer due to appearance of leading edge bubble, delicate balance between augmentation of turbulence and its decay due to local flow acceleration, separation induced transition near the mid-chord followed by breakdown on suction surface of the C-D compressor blade.

Key words: Leading edge contamination, Separation, Transition, C-D compressor blade, LES.


Leading Edge Contamination of a C-D Compressor Blade Using Large Eddy Simulation

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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